H2—O2 fuel cells use hydrogen (H2) as a fuel and oxygen (as air) as an oxidant. The hydrogen used in the fuel cell can be derived from the reformation of a hydrocarbon fuel (e.g. methanol or gasoline). For example, in a steam reformation process, a hydrocarbon fuel (such as methanol) and water (as steam) are ideally reacted in a catalytic reactor (a.k.a. “steam reformer”) to generate a reformate gas comprising primarily hydrogen and carbon monoxide.
An exemplary steam reformer is described in U.S. Pat. No. 4,650,727 to Vanderborgh. For another example, in an autothermal reformation process, a hydrocarbon fuel (such as gasoline), air and steam are ideally reacted in a combined partial oxidation and steam reforming catalytic reactor (a.k.a. autothermal reformer) to generate a reformate gas containing hydrogen and carbon monoxide. An exemplary autothermal reformer is described in U.S. application Ser. No. 09/626,553 filed Jul. 27, 2000. The reformate exiting the reformer contains undesirably high concentrations of carbon monoxide most of which must be removed to prevent poisoning of the catalyst of the fuel cell's anode. In this regard, carbon monoxide (i.e., about 3-10 mole %) contained in the H2-rich reformate/effluent exiting the reformer must be reduced to very low nontoxic concentrations (i.e., less than about 20 ppm) to avoid poisoning of the anode.
It is known that the carbon monoxide, CO, level of the reformate/effluent exiting a reformer can be reduced by utilizing a so-called “shift” reaction wherein water (i.e. steam) is added to the reformate/effluent exiting the reformer, in the presence of a suitable catalyst. This lowers the carbon monoxide content of the reformate according to the following ideal shift reaction:CO+H2O→CO2+H2.
Some (i.e., about 0.5 mole % or more) CO still survives the shift reaction. Hence, shift reactor effluent comprises hydrogen, carbon dioxide, water carbon monoxide, and nitrogen.
The shift reaction is not enough to reduce the CO content of the reformate enough (i.e., to below about 20-200 ppm). Therefore, it is necessary to further remove carbon monoxide from the hydrogen-rich reformate stream exiting the shift reactor, and prior to supplying it to the fuel cell. It is known to further reduce the CO content of H2-rich reformate exiting the shift reactor by a so-called “PrOx” (i.e., preferential oxidation) reaction effected in a suitable PrOx reactor operated at temperatures which promote the preferential oxidation of the CO with air in the presence of the H2, but without consuming/oxidizing substantial quantities of the H2 or triggering the so-called “reverse water gas shift” (RWGS) reaction. The PrOx and RWGS reactions are as follows:CO+½O2→CO2(PrOx)CO2+H2→H2O+CO (RWGS)The PrOx process is described in a paper entitled “Methanol Fuel Processing for Low Temperature Fuel Cells” published in the Program and Abstracts of the 1988 Fuel Cell Seminar, Oct. 23-26, 1988, Long Beach, Calif., and in Vanderborgh et al U.S. Pat. No. 5,271,916, inter alia.
Desirably, the O2 required for the PrOx reaction will be about two times the stoichiometric amount required to react the CO in the reformate. If the amount of O2 exceeds about two times the stoichiometric amount needed, excessive consumption of H2 results. On the other hand, if the amount of O2 is substantially less than about two times the stoichiometric amount needed, insufficient CO oxidation may occur and there is greater potential for the RWGS reaction to occur. Accordingly in practice, many practitioners use about 4 or more times the stoichiometric amount of O2 than is theoretically required to react with the CO.
PrOx reactors may be either (1) adiabatic (i.e. where the temperature of the reactor is allowed to rise during oxidation of the CO) or (2) isothermal (i.e. where the temperature of the reactor is maintained substantially constant during oxidation of the CO). The adiabatic PrOx process is sometimes effected via a number of sequential stages, which progressively reduce the CO content in stages, and requires careful temperature control, because if the temperature rises too much, the RWGS reaction can occur which counter productively produces more CO. The isothermal process can effect the same CO reduction as the adiabatic process, but in fewer stages and without concern for the RWGS reaction if (1) the reactor temperature can be kept low enough, and (2) O2 depletion near the end of the reactor can be avoided.
One known isothermal reactor is essentially a catalyzed heat exchanger having a thermally conductive barrier or wall that separates the heat exchanger into (1) a first channel through which the H2-rich gas to be decontaminated (i.e. CO removed) passes, and (2) a second channel through which a coolant flows to maintain the temperature of the reactor substantially constant within a defined working range. The barrier wall has a catalyzed first surface confronting the first channel for promoting the CO+O2 reaction and an uncatalyzed second surface confronting the second channel for contacting the coolant therein to extract heat from the catalyzed first surface through the barrier. The catalyzed surfaces of adjacent barriers oppose each other, and are closely spaced from each other, so as to define a narrow first channel through which the H2-rich gas moves.
The reformation process of gasoline or other hydrocarbons operate at high temperatures (i.e. about 600-800° C.). The water gas shift reactor is active at temperatures of 250-450° C., The PrOx reaction is active at temperatures of 100-200° C. Thus, it is necessary that the reformer, the water gas shift (WGS) reactor, and the PrOx reactor are each heated to temperatures sufficient for the fuel processor to operate. During start-up, however, a conventional fuel processor is such that the heating of various components is staged. This approach can lead to undesirable lag time for bringing the system on line. Alternately, external electrical heat sources (i.e. heaters) may be employed to bring the components to proper operating temperatures. This approach requires an external source of electricity such as a battery.
Accordingly, there exists a need in the relevant art to provide a fuel processor that is capable of heating the fuel processor components quickly to achieve these high operating temperatures for startup. Furthermore, there exists a need in the relevant art to provide a fuel processor that maximizes this heat input into the fuel processor while minimizing the tendency to form carbon monoxide. Still further, there exists a need in the relevant art to provide a fuel processor capable of heating the fuel processor while minimizing the use of electrical energy during startup and the reliance on catalytic reactions.